1. Field of the Invention
The present invention is generally related to nondestructive examination of heat exchanger tubing and more particularly to eddy current testing of such tubing.
2. General Background
In steam generators used for energy production, coolant from the nuclear reactor travels through the tubes and transfers heat through the tubes to the secondary coolant in the steam generator. Due to the harsh operating environment, it is not uncommon for tubes to develop cracking, pitting or corrosion on the inner or outer diameter of the tube that could lead to a leaking tube and mixing of primary and secondary coolant. Damage to tubes can also result from loose parts in the system. In plants that operate close to their design capacity, repair of damaged tubes to keep them in service is preferred to plugging these tubes. Installation of a sleeve that bridges the damaged area inside the original tube is a commonly used repair since it essentially maintains the heat exchange surface area. Sleeve installation for one type of sleeve is accomplished by positioning the sleeve, with a charge loaded in the sleeve, inside the tube to bridge the damaged area and firing the charge. When the charge is fired, part of the sleeve outer diameter impacts the original tube inner diameter with a force that causes a band of metallurgical bonding. This band forms a fluid seal. The expanded area of the sleeve and tube is then heat treated to relieve any stresses that could lead to stress corrosion cracking in the tube. Other sleeve installations use a roll expanded joint or a TIG or laser welded joint.
Eddy Current testing has become a standard method for nondestructive evaluation (NDE) of heat exchanger tubing. Typically a differential bobbin coil probe with the coil axis coaxial with the tube axis is used for the majority of tubing inspection.
One of the most challenging nondestructive examination (NDE) areas of the steam generator is the original tube wall behind the sleeve ends where the tube still serves as the pressure boundary between the primary and secondary side coolant, and the joint area proper (the point of expansion and metallurgical bonding between the sleeve and original tube). The parent tube area is shielded from the NDE sensor by the sleeve. Inspection in this area is further complicated by geometry signals from the sleeve end and the joint, which are ordinarily many times the amplitude of the target flaws to be detected. This makes the detection of true degradation signals more difficult. The sleeve joint presents a different set of challenges to confirm absence of flaws in the parent tube or sleeve within the weld area. This is due to the thickness of the wall created by the joint. It is also necessary to measure the diametrical profile of the joint as a process verification requirement.
A differential cross wound probe was developed to minimize the residual (unwanted) signal response caused by uniform circumferential surface discontinuities such as those at the sleeve ends or attachment locations. The cross wound probe is similar in design to a differential bobbin coil probe except that each coil consists of two 180 degree segments which are offset vertically. The second coil is a mirror image of the first coil across the center of the probe. When the probe is in a uniform circumferential surface discontinuity, any impedance change reflected into the upper 180 degree segment of the first coil will be partially canceled by a similar impedance change in the second coil segment at the same elevation. However, the differential bobbin cross wound probe still suffers from residual geometry induced noise and loss of sensitivity in areas of increased tube ID since the coil remains radially centered in the tube. This probe integrates the eddy current response for the entire circumference of the tube or sleeve at one elevation. This is due to the fact that the probe is centered as it is pulled through the sleeve and that diametrically opposed poles of each coil are substantially equidistant from the wall of the sleeve or tube being examined. The overall effect is a reduced sensitivity and higher residual noise than the current invention.
Another Eddy Current development driven by the need to improve signal to noise ratio and to provide better defect imaging was the development of a rotating pancake coil (RPC) probe technology. The RPC is ordinarily mounted on a motorized sheath which allows the coil to be simultaneously rotated and translated through the tube thereby developing a helical scan of the tube surface. The pancake eddy current coil axis is normal to the tube ID surface. The coil is mounted in an articulating mechanism allowing the coil to follow the ID surface contour, and to maintain a relatively constant coil liftoff. This inspection technology provides two dimensional defect imaging, better elimination of geometry induced signals due to the surface following characteristic, and thus better detection sensitivity for certain tube indications. However, its sensitivity to outside diameter initiated degradation in thicker or layered material is very limited.
From the above, it can be seen that a need exists for a nondestructive examination apparatus that is more effective in examining thicker walls such as that created at a sleeve/tube joint and the sleeve and original tube behind the sleeve on either end of the joint or in any other thicker or multiple wall tubing, particularly in the presence of ID geometry changes. There is also the need to measure the ID profile in sleeve attachment areas to verify proper installation.